This invention relates to ground fault protective circuitry; and more particularly to protective circuitry for sensing ground fault currents, the grounding of a neutral conductor and the shorting of a line conductor by a parallel conductive path.
In a system supplying electrical power to a load, there is always a danger that a person or animal can receive a lethal shock or be severly injured. Electrical systems supplying power to a load have traditionally been protected by overcurrent sensing devices which open the circuit upon passage of excessive current. Unfortunately, the current level which will cause injury or death to a human being is only a fraction of the current required to trip such devices. Further, property damage can result before these conventional devices have had sufficient time to respond. Of course, such injuries and property damage do not normally occur as the result of normal current flow from a system's energized conductor to its neutral conductor. Instead, they result from the unintentional flow of current from the energized conductor through a person's body or some other path not normally a part of the electrical system, to a grounded conductor other than the system's neutral conductor. Such a current flow is commonly called a ground fault.
Circuits for providing protection against such ground faults are available on the market today. A number of these circuits utilize differential transformers to sense a current unbalance in the conductors of a distribution system providing electrical power to a load, and in response to this unbalance provide a signal to remove power from the load. These available devices have, by and large, operated satisfactorily. However, some of the prior art devices are highly temperature sensitive and become unstable with temperature variations. Also, some of the prior art devices are not sufficiently immune to noise to prevent false triggering in response to noise signals.
Another problem that exists in many of the prior art differential transformer protective circuits relates to the inadvertent grounding of the neutral conductor on the load side. In power distribution systems where the neutral conductor is grounded at the source side, inadvertent grounding of the neutral at the load side may render the protective circuitry ineffective. Several systems that overcome this problem have been developed. For example, U.S. Pat. No. 3,473,091 to Morris, et al. discloses ground leakage differential protective circuitry in which an impedance is inserted in the neutral line to detect a low impedance to ground in the neutral conductor; and U.S. Pat. No. 3,663,865 to Stanback teaches the placing of a resistor across the line and neutral conductor to detect a grounding of the neutral conductor. A third patent, U.S. Pat. No. 3,611,035 to Douglas teaches the use of a tickler coil to induce a high frequency voltage on the neutral conductor to detect a low impedance to ground in the neutral conductor.
This invention provides improved ground fault protection circuitry that overcomes most of the prior art problems. In addition to protecting against ground fault, the circuitry of this invention provides protection against the inadvertent grounding of a neutral conductor on the load side. The protection against the inadvertent grounding of the neutral conductor is provided without the insertion of any additional impedance in the power lines, and without continuously placing a high frequency signal on the neutral conductor.
Of course, not all loads receiving AC power have a neutral conductor; in such systems shorting of a line conductor by a parallel conductive path can present a hazardous condition without any obvious indication that such a short exists. The circuitry of this invention, when used in such systems, detects the shorting of a line conductor by a parallel conductive path. It is also noted that with the protective circuitry of this invention the same basic circuitry may be used to provide ground fault protection plus grounded neutral protection, and ground fault protection plus protection against shorting of a line conductor by a parallel conductive path.